Add support for gpg-agent to aws-env-update.
Add support for gpg-agent to aws-env-update.

This partially fixes #7 (i.e. only aws-env-update).

Includes some monkey patching of python-gnupg in order to support gpg2
>= 2.1.3.

Expanding gpg-agent support is probably not worth it until python-gnupg
0.3.9 is released with more gpg2 2.1 fixes.

bartekj/aws-tools,bartekj/aws-tools